Stability of gas atomized reactive powders through multiple step in-situ passivation

Abstract

A method for gas atomization of oxygen-reactive reactive metals and alloys wherein the atomized particles are exposed as they solidify and cool in a very short time to multiple gaseous reactive agents for the in-situ formation of a protective reaction film on the atomized particles. The present invention is especially useful for making highly pyrophoric reactive metal or alloy atomized powders, such as atomized magnesium and magnesium alloy powders. The gaseous reactive species (agents) are introduced into the atomization spray chamber at locations downstream of a gas atomizing nozzle as determined by the desired powder or particle temperature for the reactions and the desired thickness of the reaction film.

Description

RELATED APPLICATION[0001]This application claims benefit and priority of U.S. provisional application Ser. No. 61 / 686,822 filed Apr. 12, 2012, the entire disclosure of which is incorporated herein by reference.CONTRACTUAL ORIGIN OF THE INVENTION[0002]The United States Government has rights in this invention pursuant to Contract No. DE-AC02-07CH11358 between the U.S. Department of Energy and Iowa State University.FIELD OF THE INVENTION[0003]The present invention relates to gas atomization of reactive metals and alloys, including and especially magnesium and magnesium alloys, using multiple gaseous reactive agents for the formation of a protective reaction film or layer on gas atomized powders in a manner to increase powder stability, such as increase powder thermal ignition temperature as well as reduce spontaneous discharge by spark ignition.BACKGROUND OF THE INVENTION[0004]Magnesium powder is a critical component in a wide variety of applications. Magnesium powders have application...

AI Technical Summary

Benefits of technology

[0008]In an illustrative embodiment of the present invention, an exemplary first gaseous reactive species used for the in-situ passivation of the atomized magnesium powder comprises oxygen, which can be injected either at the same level or above the gaseous second reactive species in the atomization chamber. The second reactive species is incorporated into the oxide layer (reaction product) formed on the magnesium particles by the oxygen species in an amount to modify the oxide layer in a manner that increases the thermal ignition temperature and spark ignition resistance of the atomized particles. Exemplary second species include, but are not limited to, a halogen-containing gas, such as a fluorine-containing gas. In a particular illustrative embodiment for in-situ passivation of atomized magnesium powders, the second reactive gas comprises SF6 for forming a fluorinated oxide compound as a layer covering the powder particles; however, any suitable passivating gas, such as NF3, can be considered for use in conjunction with the reactive oxygen species in this illustrative method embodiment.

[0009]The present invention envisions atomized reactive metal or alloy powders that have a thin protective layer that comprises a reaction product, such as a compound, of a metal (e.g. Mg) and the first reactive species (e.g. oxygen) wherein the reaction product, such as the compound, also includes an amount of the second reactive species (e.g. fluorine) to improve ignition stability as described above such that the atomized powders are more resistant to ignition than a simple native oxide layer covered particle to reduce or perhaps substantially eliminate the chance of powder ignition during production, handling, storage, and further processing of such reactive powders to end-use shapes. The protective layer is formed on the atomized particles to a thickness of only ten's of nanometers and yet it imparts beneficial improved resistance to ignition to the atomized powder and may also provide improved resistance to humidity during storage. The invention envisions also controlling the particle size of the atomized particles as well as to provide a relatively small particle surface area to particle volume ratio (by a generally spherical shape) in order to further reduce risk of ignition

Problems solved by technology

However, these powders are pyrophoric and must be handled carefully and kept dry at all times.

Similar reactivity issues affect the magnesium die-casting industry which has lead to extensive research on methods of protecting magnesium from spontaneous ignition during melting operations and suppressing magnesium vaporization resulting from the metal's extremely high vapor pressure.

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